Three-color photography



May 7, 1935- J. A. BALL 2,000,058

THREE-COLOR PHoToGRAPHY- Filed' June 22,' 1951 fyi?,

Pldf ABsoRPr/ON v M//vus GlEpE/y RA NGES I BLUE GREEN RED fair/sr @Sepia au Patented'Msy 7, 193s UNITED STATES- 2,000,058 THREE-COLOR PHOTOGRAPHY Joseph A. Ball,

mesne assignments,

Los Angeles, Calif., assigner, by lto Technicolor, Inc., New

York, N. Y., a' corporation of Delaware Application June 22, 1931, Serial No. 545,951

method involves dividing the light beam from the object field into as many component beams as color aspects are desired and exposing the diierent color records with the component beams respectively. Each of these methods has certain disadvantages if more than two color aspects are required, these disadvantages being well known to those skilled in the art and consisting mainly in diillculties of obtaining proper registration, sharp definition and. sufficient light intensity.

Objects of the present invention are to avoid the aforesaid disadvantages and more particularly to obtain uniform balance between successive records of the two colors (red and green e. g.) which contribute chiefly to the .body or substance of the ilnal pictures, to expose superposed emulsions with different color components of light (red and blue e. g.) for which the optical. system used has substantially equal focal lengths and, at the `saine time, to obtain maximum chromatic correction for a record of another color component (green e. g.), to obtain satisfactory color separation notwithstanding the use of a iilter of the surface-dye type which ordinarily can not be produced with uniformity in every respect, simultaneously to expose emulsions in juxtaposed apertures without substantial fogging due to scatter light reilected from one emulsion to the other or from other surfaces, and generally to improve the art of three (or more) color photography, particularly for motion picture use.

Further objects are to provide a green color separation record of maximum sharpness, to eliminate the effects of chromatic defects of the optical system employed for exposing the negatives, to avoid the detrimental inuence of light reilected or scattered from certain elements of the optical system, to provide an arrangement which does not necessitate the use of special and abnormal emulsions and permits color separation without special sensitizations and which permits the incorporation of a key record without susbtantial change in the three color system. Stillurther objects are to avoid di'iliculties inherentv in color filters which are integral with motion picture. films, and generally to provide improvedoptical arrangements and methods which permit eillcient commercial production of motion picture illms under conditions which are as nearly as possible similar. to those required for the taking oi ordinary motion picture negatives. Additional objects of my invention will be apparent from the following explanation accompanied by a drawing in which: 5

Fig. l is. a diagrammatic side view of one embodiment of the present invention;

Fig. 2 is a similar view of a modification;

Fig. 3 is an axial section of another embodiment;

Figs. 4 and 5 are side elevations of the embodiment shown in Fig. .3, viewed from the righthand side and rear end of Fig. 3, respectively;

Fig. 6 is a curve showing the focal lengths of a lens system for different wave lengths, the socalled secondary spectrum; and

Fig. '7 is a schematical drawing indicating the absorption ranges of certain filters employed in the present invention.

Herein the terms blue, green and red ranges of the visible spectrum and corresponding terms are not strictly conilned to the ranges having these hues but connote the three main regions of the spectrum namely those of low, medium and high wave lengths; the terms are not intended to denne deilnite limits of these ranges. Y

The subject matter oi this invention relates to that type of color photography where the image bearing beam is divided by suitable light-dividing means into Itwo components, one of these components being used to expose superposed emulsions (of a bipack e. g.) and the other component being used to expose at least one other emulsion, as described in my Patent No. 1,889,030, of November 29, 1932, for multicolor photography with bi-packs, of whchthis is in part a continuation. A characteristic feature 0f this arrangement consists in providing one of said nrst two emulsions substantially like the third emulsion, both emulsions being preferably panchromatic and cut from the same stock. By forming on these like emulsions the two images (e. g. the red and the green) which contribute mainly to build up the gradation or, in otherwords, supply the body" or substance of the y final picture (the gradations of the blue record, which in a subtractive print appear as gradations of yellow dye, being rather weak) and which must therefore be carefully balanced in exposure and development, the desired balance may be obtained uniformly and with facility. riis balance can not be obtained by recording the red and green aspects in the same light path for various reasons, chiefly because it has thus '55 illtering, it would be radically different from the red sensitive film and the two could not be properly balanced in exposure and development throughout varying conditions such as are encountered in cinematography.

It is well known that in cases 1where dierent color aspects are exposed with a single lens in a single focal plane, the lens must produce images which are identical as to size, conguration and definition. @ne of the most serious diiiiculties in designing a photographic lens with a view to these requirements is the so-called secondary spectrum caused. hy the chromatic aberration of the normal lens which has not a single focal piane for all colors but diiere'nt focal planes for different wavelengths, the focal planesicorresponding to the lower wave lengths ordinarily beingr closer to the lens than those corresponding to the higher Wave lengths. With the ordinary means known to the lens designer it is possible to design a high speed lens which has exactly the same focal lengths for any two selected wave lengths. In other words. rays of these twoselected wave lengths will be brought to form images in the same plane but other wave lengths will form vimages on either side of this common focal plane. It is furthermore known that the color separation nega-tive which `records the medium or green range of the visible sctrum contributes primarily to the definition of the ilnal positive and should therefore be as sharp as possible, especially as it may be used for making neutral key prints as proposed in my copending application Serial No. 519.721, illed :March 3, 1931.

According'to the present and blue invention, the red aspects are recorded on emulsions which are substantially in the same plane, with a lens system' having a secondary spectrum as shown in Fig. 6 in which the deviations of the focal lengths for the various wave lengths ci' the solar spectrum. from an arbitrary selected focus. are plotted against the waye lengths, the three principal ccdor ranges also being indicated in this ligure.` From the curve it can be seen that the green range has a substantially horizontal tangent to which each point of this range of the are not so well corrected is of minor importance since these ranges do not contribute largely to the denition of the positive color reproduction. However, the average focal lengths, or the focal lengths of the centers oi these marginal ranges, substantially coincide, resulting in substantially identical'focal distances for the blue `and red records which, according to the present inven- YNo. 18,680 oi December the other beam. YSince each oi' the recording elements will. if permittedfrefiect trom itssurface to the other element light of all wave lengths including that which the other element is to record, and since light may be scattered from other surfaces into the recording apertures, I propose to obstruct this reflected and scattered -lightfrom the "apertures For this purpose certain surfaces of the optical system are made opaque as will be described in` detail hereinciter,l and furthermore the filters which are arranged in front of the apertures cooperate in excluding anylight which is not to be-reccrded in the coordinated aperture. Hence, a.V minus green filter is inserted in front ci' the red and blue recording aperture, in addition to the green filter in front of the green recording aperture.

The minus green filter has stillanother iinportant function, made possible by the peculiar general arrangement of the invention. As will be described later on in detail, a red lter is inserted intermediate the blue and red recording emulsions, this red lter moving with the two films and being for example appliedas a surface dye upon the blue recording front enmlsion,`

as for instance disclosed in the Reissue Patent 6, 1932, of Leonard T. Troland for Color photography. In applying Ve. iilter layer to a photographic emulsion it is practically impossible to maintain an exact limitation o1' the absorption range (or the cut of the filter as it is sometimes called) uniformly over the entire length of the film strip. For this reason, the color selection as recorded upon the nlm behind this lter would uctuate and impair the quality of the final positive. This is etrectively prevented-by the minus green niter, as indicated in Fig. 5 which shows the absorption ranges of the red and minus green iilters.

As indicated by the dotted end of the line corresponding to the absorption range-of the red filter at V, the cut oi' the red filter may vary within the absorption range of the green lter without causing any detrimental effect, since the end of the red admission at W is determined only by the stationary minus green niter.

In order that the invention may be more easily understoc-d, I will now describe the specific embodiment shown in Fig. l where O represents an optical system of any suitable construction, and P a light splitting device comprtsing a partly reecting and partly transmitting surface M which divides an incident light beam L into two divisions S and R as indicated by arrows. I, 2 and I are suitablesupports, for instance, cellulose iilms, -bearing emulsions Il, Il; Il is an ordinary silver bromide emulsion, blue sensitive only, requiring no special treatment and oi' a kind which can easily be obtained with the required thin and transparent chiuacteristics 'for the front nim o! a bipack; and I! and I3 are ponchromatic emulsions -of usual characteristics.

LRDrepresentsalteninthisinstaneensux-nce the three records of .a three-color photographic process a fourth so-called key print, usually reproduced in gray vor black which has the main purpose of giving definition to the colored picture and in certain cases to supply the deep shadows. My invention lends itself wel! for the production of colored pictures with key prints and a four-color system of this kind will now be described, referring especially to Fig. 2. O, P and M is again an optical system and I, 2, 3 and 4 are supports, the fourth support 4 having been added to I, 2 and 3 of Iig, 1. Emulsion I4 upon support 4 may be a panchromatic emulsion which is thin and transparent enoughto transmit as much light as possible to the back emulsion I3. A green surface dye GD is for example applied to emulsion I4, having the purpose of admitting to emulsion I3 only the radiations suited for recording thereon the green aspect,l

the surface dye filter GD replacing separate lter G of Fig. .1 Y is a yellow filter which absorbs light in the same manner as filters which are used withv orthochromatic emulsions for making ordinary black and white pictures.

Emulsions II, I2 and I3 are ofthe same nature as the correspondingemulsions of Fig. 1; I2 and I3 being substantially identical panchromatic emulsions, preferably slittings from the same stock, and II an ordinary blue sensitive transparent emulsion. It is of 'course necessary that the key print be as definite in sharpness as possible and that the deflnitionbe preserved through the process. For this purpose it is desirable to print key positives by means of contact printing which does not cause loss of definition and does not build up graininess. With the arrangement shown in Fig. 2 it is possible to use contact print- Y ing in spite of the fact that the reflected image is reversed, because the key emulsion is exposed through the back, which counters the reversal of the image by reflection.

A further advantage of v this arrangement arises from the fact that it is desirable to have for the purpose described key prints of a gamma of approximate unity, kthe same gamma commonly used for photographic sound records. In making sound motion pictures the sound track is therefore printed together with the key print, thereby making unnecessary any differentiation in the character of picture and sound records. The negative sound record (ordinarily made on a separate negative) may be printed on the positive film along with the aforesaid key pictures and this combined print developed to approximately gamma unity after which the film may be fixed, washed, dried and the color aspects applied by imbibition.

According to my method of recording the red and green aspects in different light paths, I am enabled to use substantially identical emulsions of a normal type for these two aspects, and (not considering the key record) require a special emulsion only for the blue aspect, this emulsion being special only in its transparency and in its surface coating. I am also enabled to balance, with uniformity and facility, the color values of the two records which are most important and most difficult to balance.

Still another advantage of having the red and green-records on identical emulsions and therefore well balanced,- arises from the fact that the human eye is much less sensitive to variations of blue and yellow light than to variations of red and green light. This circumstance is probably due to the fact that various sources of artificial light and also daylight vary a great deal in their blue components, that is to say, they vary from blue-white to yellow, depending upon the greater or lesser intensity' of the blue component as compared to normal. Normal is usuallytaken to be noon sunlight, in which case north sky-light is considerably more blue; late afternoon sunlight is considerably more yellow; highintensity arc light generally more blue; and incandescent light very much more yellow than normal. For this reason it does not matter much if the blue "component of a color motion picture film is not particularly well balanced with respect to the red andgreen records. However, the red and green records have to be well balanced as the eye is very sensitive to changes of these colors'and irregularities of the relative values of red and green are therefore to be avoided. The arrangement of the present invention with like emulsions for the red and green color aspects takes care of these facts in a simple manner and assures the proper balance of colors also with respects to this phenomenon.

Referring now to Figs. 3,v 4 and 5, with a lens,

system having a secondary spectrum according to Fig. 6, the blue and red aspects are recorded on emulsionsII and I2 substantially in the same plane, the plane of emulsions II and I2 corresponding to the focal plane FM of Fig. 6 and the plane of emulsion I3 corresponding to focal plane FC of Fig. 6. By this arrangement the red and blue images are recorded sharply upon the bipack at the same focus setting and, since the minimum focal length is in the green,` the f maximum chromatic correction is obtained in the green which is desirable for reasons explained above. The difference between the two focal planes can be easily adjusted by varying the distance of the emulsion planes from the prism faces 2| and 22, as for instance by inserting cover glasses of different thickne between thev prism faces, or the filters G and MG, and the films. As shown in Fig. 3, the filters may be amxed directly to the prism faces, MG being the minus green filter and G the green filter. As mentioned before, the film in the aperture reflects light of all colors which have been allowedto fall on it, including light of the color to be recorded lat the other aperture; and this reflected light, if allowed to reach the film' in the other aperture, would fog that film. 'I'his fogging tendency .is most pronounced at the sides of the apertures which are close together, namely, near the edge formed by prism faces 24| and 23. Furthermore, the prism faces reect light as indicated by dotted line LR. The green light which is reflected from the film 3 is effectively excluded from the other aperture by the minus green filter MG, whereas the minus green light reflected from film I is eliminated by green filter G. In order to shield the apertures against.

the especially pronounced reflection in the coropaque either by blackening the adjacent marginal'parts of 2I and 23, or by making the mar- `ner between faces 2l and 23, this corner is made l gins of the mirror plane M opaque or, as shown in the drawing, by providing a notch with blackened faces 24 and 2l. In order also to exclude the light reflected at surface 22 (as indicated by ray LR) another opaque edge or notch with blackened faces 26 and 21 is provided. These notches have the further advantage of providing space for screws or posts 3| and 32 which connect yoke 34 with base plate 33, clamping the prism hletvveen` these construction elements.

This arrangement permits the utilization oi' the optically beneficial notches for a mechanical design which makes the prism mount compact so that it can be easily confined in the restricted space available for it in the camera.

Figs. 3 and 4 also show a shutter 3U of conventional design, arranged intermediate the lens system O and the prism P. Figs. 3 and 5, in connection with Fig. 7, further explain the cooperation between the red filter RD intermediate the emulsions Il and I2, and the minus green filter MG. As discussed above, the filter RD, which should be integral with one of the bipack films and determines the color selectivity of the rear emulsion i 2, can not readily be made uniform throughoutlengths of film. This causes manufacturing difficulties so that specifications concerning the transmission qualities of the filter layer can not always be met. However, with the arrangement according to Fig. 3, the requirements concerning the red filter can be less exacting because the minus green filter MG determines the limit towards the green range of the wave lengths of light recorded, as the red color aspect, upon emulsion I2. i

It is very important exactly to maintain this limit because it is on the one hand desirable to admit as much light as possible to the red recording back film in order to obtain the speed necessary for motion picture work whereas, on the other hand, if the admission range of the red filter is widened beyond a certain wave length, the color separation degenerates. Therefore if the out of the red taking range is exactly determined by a fixed filter (viz. minus green filter MG) the limit of optimum efficiency for both requirements can be continuously maintained.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. Photographic apparatus comprising juxtaposed apertures, one for a green sensitive film and another for superposed films sensitive to blue and red light respectively, means for focusing light at the apertures respectively, a green filter in the light path to the first aperture, and a minus green filter in the light pat-h to the second aperture.

2. Photographic apparatus comprising a lens system having substantially equal focal lengths for spaced color ranges and a different focal length but maximum axial chromatic correction for an intermediate range, means for dividing the beam of said system into two component beams, means for recording said intermediate range in one component beam and means for recording said spaced ranges on superposed emulsions in the other component beam.

3. Photographic apparatus comprising a lens system having substantially equal focal lengths for thered and blue ranges and a different focal length but maximum axial chromatic correction for the green range, means for dividing the light from said system into two component beams, apertures in said component beams, identical panchromatic emulsions inthe respective apertures and in one aperture a transparent emulsion in front of the panchromatic emulsion, and a filter in front of the latter aperture excluding light of a color to which at least one of the films at that aperture is sensitive.

4. Photographic apparatus comprising a lens system having substantially equal focal lengths for the red and blue ranges and a different focal length but maximum axial chromatic correction vfor the green range, means for dividing the light from said system into two component beams, a green filter in front of one of said apertures and a panchromatic emulsion behind said green filter, a minus green filter in front of the other aperture and superposed in the latter aperture a transparent blue sensitive emulsion and a panchromatic emulsion.

5. Photographic apparatus comprising a lens system having substantially equal focal lengths for the red and blue ranges and a different focal length but maximum axial chromatic correction for the green range, a reflector behind said lens system for splitting the light into direct and re-l flected light components, a green filter in the direct component, a minus green filter in the reflected component, a panchromatic emulsion behind the green filter, a panchromatic emulsion behind the minus green filter, atransparent emulsion intermediate said minus green filter and said second panchromatic emulsion and a red transmitting, blue absorbing filter intermediate said transparent emulsion and said second panchromatic emulsion.

6. Photographic apparatus comprising a lens system having substantially equal focal length for the red and blue ranges and a different focal length but maximum axial chromatic correction for the green range, a partial reflector behind said lens system for splitting the light into two component beams, said reflector comprising a glass block with a diagonal partial reflecting surface, the corners of the block adjacent said surface being notched and mounting-means arranged in said notched corners, a green filter in one component beam, a minus green filter in the other component beam, a panchromatic emulsion behind the minus green filter, a transparent emulsion intermediate said minus green filter and said last panchromatic emulsion, and a red transmitting, blue absorbing filter intermediate said transparent emulsion and said last panchromatic emulsion.

7. In the art of color photography the method of recording' the blue, green and red color aspects of an object field which comprises producing an image bearing beam with an optical system having substantially equal focal lengths for the red and blue spectral ranges and accurate axial chromatic correction for the green range, dividing the beam into two cornponent beams, recording with one component beam the green aspect of the object field upon a panchromaticl emulsion, recording with the other component beam the blue aspect upon a transparent emulsion, and recording with the same beam the red aspect upon an emulsion like that used for recording the green aspect and placed behind said transparent emulsion.

8. In the art of color photography the method of recording the blue,d green and red color aspects of an object field which comprises` producing a light beam bearing images of the red and blue aspects of substantially equal axial chromatic correction and an image of the green aspect of maximum chromatic correction, dividing the beam into two component beams, one

Abeam being reflected and the other transmitted without reflection, recording the green aspect `of the object field upon a panchromatic emulsion in one beam and recording vthe blue and transmitted and reflected beams, recording the` red aspects with the other beam on superposed emulsions.

9. In the art of color photography the method of recording the blue, green and red color aspects of an object field which comprises producing a light beam bearing images of the red and blue aspects vof substantially equal axial chromatic correction and an image of the green aspect of maximum chromatic correction, dividing the beam into two component beams, one beam being reflected and the other transmitted without reection, recording the green aspect of the object eld upon a panchromatic emulsion in the direct beam and recording the blue and red aspects with the reflected beam on superposed emulsions.

10. In the art of color photography the method of recording the blue, green and red color aspects of an object eld which comprises producing van image bearing beam with an optical system having substantially equal focal lengths for the red and blue spectral ranges and substantially l.perfect axial chromatic correction for the green range,..dividing the beam into two component beams, recording the green aspect of the object field upon a panchromatic emulsion behind a green filter with one beam, and recordingthe blue and red aspects on superposed emulsions behind a minus green filter with the other beam.

11. In the art of color photography the method of recording the blue, green and red color aspects of an object field which comprises producing an image bearing beam with an optical system having substantially equal average :focal lengths for the. red and blue spectral ranges and more accurate axial chromatic correction for the green range, dividing the beam into two component beams, one beam being reflected and the other transmitted without refiection, recording the green aspect of the object ield upon a panchromatic emulsion behind a green yfilter with the transmitted beam, recording the blue aspect upon a transparent emulsion behind a minus green lter with the reflected beam and recording with the latter beam the red aspect upon an emulsion like that usedfor recording the green aspect and superposed behind said transparent' emulsion.

l2. The art of color separation photography which -comprises producing an image bearing beam with an optical system having substantially equal focal lengths for the red and blue ranges a'nd substantially equal focal lengths for the margins of the green range, dividing the beam into two component beams, recording the green vaspect with one component beam and recording the blue and red aspects in superposed juxtaposed emulsions with the other component beam.

13. In the art of color separation photography the method of obtaining a green record of maxi- `mum deiinition which comprises producing an image bearing-beam with an optical system having substantially equal focal distances for the center portions of the red and blue ranges and maximum axial chromatic correction for the green range, dividing the beam into directly green aspect with the direct beam and recording the blue and red aspects with the reflected beam on superposed emulsions.

14. In the art of color photography the method of recording the blue, green and red color aspects of an object field which comprises dividing the light from the field into two component beams, with one beam recording the green aspect behind a -gr'een filter and with the other beam recording the blue aspect on a blue sensitive emulsion behind a minus green filter and the red aspect behind said minus green filter and a red transmitting blue absorbing filter the blue and red recording emulsions being superposed.

l5. In the art of color photography the method of recording the blue, green and red color aspects of an object field which comprises dividing the iight from the field into two component beams, one beam being reflected and the other beam directly transmitted, with the direct beam recording the green aspect behind a green lter upon a panchromatic emulsion and with the reflected beam recording the blue aspect behind the minus green filter and the red aspect behind the minus green filter and a red transmitting, blue absorbing filter upon a panchromatic emulsion like the green recording emulsion the blue and red emulsions being superposed.

16. The method of cinematographically recording two color ranges upon two superposed emulsions which comprises exposingl consecutive picture areas of the lm by excluding the spectral ranges not recorded on either emulsion with a filter having an unvarying absorption range, andexcluding the light not to be recorded on the back emulsion with a filter having an absorption range which varies, for different areas, within the range of the first filter.

17. The method -of cinematographically recording two color .ranges upon two superposed emulsions respectively which comprises exposing consecutive picture areas of the film by excluding the spectral ranges not recorded on either emulsion with a filter having a definite absorption range, in front of both emulsions, and excluding the light not recorded on the back emulsion with a'iilter progressing with the emulsions intermediate thereof and having an absorption margin varying, for different areas, Within the range of the front lter.

18. Apparatus for recording the blue and red color ranges upon two superposed emulsions which comprises a minus green front filter for excluding the green range, and a red filter intermediate the two emulsions for excluding the minus red light.

19. Apparatus for recording the blue and red color ranges upon two superposed emulsions which comprises a minus green front filter for excluding the green range and a red filter intermediate -the two emulsions for excluding the minus red light, said red lter having an absorption range whose margin towards the green side lies within the absorption range of the minus greenl front filter.

20. In the yart of color photography the method 'of recording the blue, green and red color aspects of an object field which comprises dividing the light from the iield into two component beams, recording thegreen spectral range on a panchromatic emulsion in one component beam, recording the minus green range on films including a panchromatic emulsion in the other component beam, andexcluding the entire minus green range from the green recording beam andthe entire green range from the minus green recording beam, thereby reducing detrimental effects of scatter light deviated from one beam into the other beam.

21. Apparatus for recording the blue and red color ranges upon two superposed emulsions 6 i c which a substantielly minus green front filter for substantially excluding the green range from said emulsions, and a substantially blue absorbing nlter intermediate the two emul;

Y mitted by said blue recording emulsion, lsaid blue absorbing lter having an absorption range whose'margin towards the green. side lies witmn the absorption range of -thefnclinus green front muereY 23. The method of: cinematographically recording two color ranges upon two superposed illm strip emulsions which comprises Yexposing 'iconsecutive picture areas of the emulsions by excluding the spectral lranges not recorded on either emulsion with a stationary illter in front i o! both emulsions,Y and excluding Ythe light range not to be recorded on the'back emulsion with a 'surface ilIter on one of said emulsions, and having an absorption range whose margin towards said first mentioned ranges lies within the absorption range ofrsaid stationaryfiilter.

JOSEPHA. BALL. 

